If one's parents are sinners, how can that person obey them ? What are the boundaries of obeying parents ?
The commander of the faithful (P.B.U.H) has said : " Goodness towards (one's) parents is the greatest obligatory act." Mizanul Hikmah, Volume 10, Page 709
Imam al-Ridha(P.B.U.H) has said:" Allah, The Mighty, The Glorious, has commanded three things with which he has associated three other things. He associated payment of zakat along with prayers such that the one who offers prayers , but desists from offering zakat, his prayers are rejected. He associated gratitude to him with gratitude towards parents such that one who is grateful to Allah, but ungrateful to his parents , will be deemed as ungrateful to Allah. He associated righteousness with bonds of kinship such that one who is righteous, but has severed relations with his kin, will be deemed as not being righteous." Biharul Anwar, Volume 74, Page 77
so it is not our headic to concern of our parents sin and mis behave with them we should pray for them to ALLAH TO FORGIVE THEIR SIN